The Law itself can be divided into two categories: the ritual law and the purity law. The ritual law is concerned primarily with sacrifices and the duties of the priests and Levites, and as such is clearly inapplicable to Christians. The purity law, which addresses all of the different ways the Jews were to conduct themselves so as to remain distinct from the pagan nations that surrounded them, is a bit trickier. On the one hand it contains all of the dietary laws, which the apostles specifically declared were non-binding to Gentile believers, but it also includes many commands that we would agree with today (don't defraud your neighbor, don't sleep with your mom, don't sacrifice your children to Molech, etc.).
As a result of this apparent dichotomy, conservative theologians created a third category: the moral law. While their rationale for doing so doesn't come completely out of thin air, their reasoning seemed somewhat arbitrary to me even in my days as a "don't rock the boat" student at a Christian college. Unlike the ritual law and the purity law, the moral law can only be discerned from the context of the Pentateuch if one limits it to the Ten Commandments.
And even the Ten Commandments aren't held to strictly, since many Christians believe that the New Testament releases them from all but the most figurative implications of the Fourth Commandment (keep the Sabbath). While the meaning of the word "adultery" in the Seventh Commandment is widened far beyond its usual meaning of dealing specifically with cheating on one's current wife, numerous exceptions are imagined for the Sixth Commandment (do not kill) and the Fourth Commandment is reduced to a purely spiritual and/or allegorical meaning.
Aside from the Nine and a Half Commandments, the "moral law" is largely the result of cherry picking those commands that still seem like good ones to follow, even when they're surrounded by other commands that we no longer pay any attention to. Some would contend that being quoted in the New Testament is sufficient grounds for being included in the moral law, possibly making some allowance for the context of how or why it's actually quoted, but that still leaves us with a pretty patchy list of verses and not all of the ones theologians like to include. So in short, the moral law is whatever a particular theologian says it is.
Not that this has stopped theologians from trying to create a more solid-sounding basis for defining what constitutes the "moral law." Professor James DeYoung, in his otherwise impenetrable book on the subject of homosexuality, makes the following rather breathtaking claim:
"Leviticus 18 and 20 condemn homosexuality, whether consensual or as rape. The contexts of chapters 18-20 are universal and transtemporal in nature. They apply to all peoples and for all times. If the exhortation to "love your neighbor as yourself" (Lev. 19:18) is universally true, so is the prohibition of homosexuality." (page 61)
Few conservatives would argue with his first sentence, of course, but the remainder of his assertion is nothing short of jaw-dropping given what it places into the category of universal moral law (19:19-28):
-Don't plant two types of seed in the same field
-Don't wear clothing made from mixed fabrics
-Sleeping with a betrothed slave girl is less severe than other forms of adultery and only requires a sacrifice
-Don't harvest fruit from a new tree for the first three years; all fruit from the fourth year is to be given as a sacrifice
-Don't eat meat with blood in it
-Don't trim your beard or sideburns
-Never get a tattoo
While most of the other commands in Lev. 19 can arguably be said to be universal in scope, there's nothing in the context of the chapter to adequately separate the above commands from the others (which surround and intermix with them), and DeYoung doesn't even seem to notice that these are there. That a published Bible scholar with a Ph.D. and a teaching position at a seminary could make such an assertion doesn't do much for my faith in conservative Christian scholarship.
I suppose one could place parentheses around verses 19-28 and try to argue that it's an irrelevant tangent in the middle of the moral law (though doing so would require discarding a command against divination), but that only strengthens the case for considering chapters 18 and 20 separately from 19, given that chapters 18 and 20 both begin and end with commands not to emulate the practices of the Egyptians and Canaanites, and that nearly all of the activities listed in those chapters are known to have occurred within the religious practices of those two cultures.
Without getting too far into the debate over exactly what Leviticus 18:22 prohibits (though that is a very interesting study), there simply isn't a solid textual case to be made for removing chapters 18 and 20 from the purity law and relabeling them as moral law. The best conservatives can argue in their defense is to assert that to fail to place those chapters within the moral law is to condone incest and bestiality, since those prohibitions are also found there (though, interestingly, nobody seems too concerned about the prohibition against sleeping with a woman during her period, even though it falls right in the middle of the chapter). On the other hand, if it's really that difficult to come up with strong reasons for opposing incest and bestiality apart from those verses, we've got bigger issues than how to categorize our ancient texts.
In the final analysis all of this picking and choosing seems, to me, to be no less disrespectful of the Old Testament than saying that it's completely inapplicable to modern life. Would the Jews that these books were written for have recognized this distinction? Hardly; whether or not they would have objected to the use of "ritual law" and "purity law" as descriptive labels, to them every last word, syllable and letter of the Law was part of the "moral" law, whether one was a Jew or a Gentile. So are we, or are we not, as Christians, still under the Law?